I would have to say that Unreal Tournament is the coolest gun game series. I especially like UT 2004, as i has numerous game modes, like:
-CTF
-Onslaught (Control Nodes throughout the map, take control of a node connected to the enemy base to make it vunerable to destroy. Vehicles included)
-Assault (One team attacks and tries to complete objectives, while the other team defends)
-Invasion (Your team has to defend against waves of computer controlled aliens)
-Double Domination (Your team has to control 2 points on the map for a certain amount of time to score a point)
-Bombing Run (A sport like game where each team has a goal at their base. Take the ball to the opposing teams goal to score)
Also, with some crazy weapons like the Redeemer, that fires off a small nuke that obliterates everything.